A major use of e-beam testers is the stroboscopic extraction of voltage waveforms on integrated circuits. A pulsed beam of electrons is directed at a test pad connected to a selected node of the circuit to be tested. Excitation by the beam of electrons causes a number of electrons to be returned with an energy proportionate to the voltage signal appearing on the test pad at the instant the electrons strike it. The energy of the returned electrons can then be analyzed electronically and the voltage signal appearing on the test pad can be constructed.
A conventional test setup for stroboscopic e-beam testing is shown in FIG. 1. A test device driver 12 provides test pattern to the inputs of device 16 under test. The test pattern normally repeats in a loop. In this example, only four inputs A, B, C and D are shown; however, the number is variable depending on the needs of the user and the capabilities of the test equipment. The test device driver also creates a trigger with a specified time relationship with the signals being provided to the inputs of the device under test. The trigger normally is associated with the start of each new loop of the test pattern. The trigger is provided to the blanking mechanism of the e-beam tester electronics and e-beam generator 38 through a variable delay circuitry 30 to generate electron pulses 52.
The e-beam tester electronics and e-beam generator 38 are directed at a selected test pad on the device under test 16, upon which a test response signal will appear in response to the various test input signals applied to the inputs of the device. A delay is selected using the variable delay box such that the e-beam tester electronics and e-beam generator 38 will sample the signal appearing on the test pad at a selected period of time in relation to the trigger. In response to a series of events triggered by the trigger, the e-beam tester electronics and e-beam generator 38 sample the signal on the test pad for the time defined by the variable delay circuitry 30 by directing a pulsed beam of electrons 52 at the test pad and analyzing the returned electrons 54. This process is repeated for a preselected number test pattern loops such that a corresponding number of samples are taken. The variable delay is then stepped a number of times such that the signal appearing on the test pad is sampled at a number of points with differing time relations to the trigger. Thus, by defining time zero by the trigger, varying the delay, and sampling various points in time of the test response signal, a composite waveform of a time variable test response signal can be created.
A problem arises in that an uncertainty is introduced into the test by an "insertion delay" which adds to the overall delay between the output of the trigger from the test device driver, and the e-beam striking the test pad. This creates uncertainty as to where a test response signal stands in relation to the point in time defined by the trigger, normally the start of a loop (time zero). This insertion delay is in addition to the controlled delay supplied by the variable delay box, and is caused by random and unknown factors in the system. During the time between the issuance of the trigger by the test device driver and the e-beam striking the test sample, typically the trigger will be passed through trigger circuits, analog and/or digital delay generators, pulse forming and shaping circuitry, and uncalibrated lengths of cable. Some elements of the insertion delay may likely depend on such factors as trigger rate and the delay scan range.
Similarly, an insertion delay between the test device driver and the device under test may further add unknown delays into the system. The varying of the test setups used between different devices under test may vary, and this variation is usually difficult if not impossible to measure. In this case, if the insertion delay between the test device driver and the device under test is large enough, the electron pulse beam generated from a trigger at the beginning of the test pattern may arrive at the test pad before an expected signal arrives.
The circuit designer has a critical need to understand the timing relationships between the test pattern input to the device under test, the trigger which normally defines the start of a test pattern loop, and the test response signals appearing on the test pads. If variable and unknown delays modify the relationship between the trigger, and either the test pattern input to the device under test or the resulting signals appearing on the test pads, the circuit designer cannot fully appreciate the operation of the device under test. During the testing of the interior of a large and complex integrated circuit using a complex test pattern, the lack of knowledge of the insertion delays can lead to unnecessary confusion as to the meaning of the results.
The above testing artifacts have become more serious as circuit speeds have increased.
Thus, a need has arisen for a method of accounting for these insertion delays to better allow the circuit designer to understand the time relationship between the trigger defining the start of a test pattern loop and firing the e-beam tester, and the signals appearing on the test pads.